All very true. The problem is owners used to adjust to more than meet the original lamp load and now try to match larger lamps, cooking the dynamo. With original wattage lamps and precautions at lengthy traffic stops the 1930s cars could be driven any distance at night.
A characteristic of the unregulated system is that as voltage falls, so does dynamo output, so can drive on to trouble. If caught far from home with an inadequate system, can disconnect one lamp! May get better lamps with lower wattage bulbs!

I dunno about pre 1933 cars, which Vince will hopefully answer. But If you look at John Cornforths Dorset Club circuit diagram, if the wiper and petrol gauge are directed to earth somewhere else, then the lead from the cutout E serves only to earth the shunt voltage winding. So the extra resistance can be placed externally between the E terminal and earth. The resistance is given as 30 ohms and the added should be the same (or 33, more available).

If no resistor is added to the field circuit, esp care necessary to keep the third brush as retarded as possible for required (modest) output.

Note that the 10 amp or whatever net output rating of the dynamo does not refer to 12v operation, when a lot of hidden current may be disappearing into the field !

Location: Auckland, NZ

Frightening fact I have been driving mine for 50 years!!!!!!

Location: The Centre of the Universe

As I said Nick I am a relative newcomer so still learning

Location: NZ